1. Field of the Invention
The present invention relates to copy protection of video material by embedding robust identification codes (e.g., watermarks or fingerprints) in video signals, and use of these identification codes for a xe2x80x9ccopy-oncexe2x80x9d method and apparatus.
2. Description of the Related Art
Copy protection of video material, for instance movies on video tape, is a well-known problem. A solution for conventional analog video taped material is the well-known Macrovision anticopy process, which adds pulses to the vertical blanking interval of the video signal (see Ryan U.S. Pat. Nos. 4,631,603 and 4,695,901, incorporated herein by reference). These added pulses confuse the automatic gain control circuitry of a typical VCR (video cassette recorder) and thereby make any recording of the video signal unviewable, by disrupting the picture.
Copy protection is an even more important problem with digital video, because by its nature a digital recording of a video signal is an exact replica of the original, and unlike analog recording, suffers no degradation. Hence with digital video recording there is even more incentive for people to make unauthorized copies of copyrighted video material. This has been a major impediment to the introduction of prerecorded digital video media and to commercial introduction of digital video recorders.
One known solution to this problem is use of watermarks. Watermarks are digital codes embedded in a video signal which can be read by a reader (detector) present in a compliant video recorder or player, where the watermark itself does not visibly interfere with the video signal (i.e., there is no significant degradation). The watermark is read by the detector circuitry in the recorder or player which instructs the recorder or player to perform certain functions, such as do not copy, or allow only a single copy of the material, depending on the watermark. Use of watermarks requires special detector circuitry in a compliant recorder or player.
Watermarks are special signals that are built into the video, so they cannot be stripped out without substantially damaging the video signal; it is difficult to remove a watermark deliberately or accidentally. Watermarks are a so-called bilateral copy protection scheme that require a specially adapted (xe2x80x9ccompliantxe2x80x9d) recorder/player which can detect and respond to the watermark. A conventional non-compliant recorder ignores the watermark and hence the watermark will have no effect. This to be contrasted with the above-described xe2x80x9cunilateralxe2x80x9d Macrovision anticopy process which however in general is not suitable for protecting a digital signal.
While such watermarks are useful and have already been introduced to a limited extent in commercial applications, they have significant drawbacks. The most important drawback is in the copy-once situation, typically encountered in the cable television industry with pay per view broadcasting, for instance of movies. The cable television industry has created an expectation amongst its subscribers that they are allowed to make their own personal recordings, using a VCR, of the cable transmission of for instance a movie. While this single recording by a user for his personal use has apparently generally been found acceptable by both copyright owners and the cable television industry, the copyright owners (e.g. movie studios) do not wish to allow any second generation recordings to be made. Hence it is desirable to allow a copy-once video transmission, for instance via cable television, while prohibiting any second generation copies of the first copy.
Prohibition of such second generation recordings is possible using conventional watermark technology. Typically, for this situation, there are two classes of watermarks: a xe2x80x9ccopy-neverxe2x80x9d watermark and a xe2x80x9ccopy-oncexe2x80x9d watermark. Both must be read by the compliant video recorder doing the copying, which changes the copy-once watermark to a copy-never watermark upon recording. A problem with this is the extra cost in the compliant recorder, since it must be able to detect both of the two different watermarks and also add (write) the copy-never watermark. Such a system, in addition to being relatively expensive to implement, is also easily subject to technical defeat, and hence is inadequate.
There is a need for a copy-once method in the digital video recording field that offers improved economics and security over the existing art. There is also a need for a copy-once method that requires only one watermark and therefore only one watermark detector per compliant recorder (or player) apparatus. Such a method needs to have no watermark adders or modifiers in compliant recorders. (xe2x80x9cCompliantxe2x80x9d here means an otherwise conventional apparatus that has been modified by additions of special circuitry and/or software in accordance with the invention.) Manufacturers of video players and recorders will add the complying circuitry and software to their products if they wish to practice the invention, and suppliers of video material will similarly add the required signals to their programs.
Additionally such a system should be simple to execute, require no secrets to be maintained to ensure its security, and operate in both the analog and digital domains. A further requirement is that even if the first (permitted) recording is made on an existing conventional consumer VCR, whether VHS, S-VHS or 8 mm (which normally could not be expected to comply with a digital copy-once protocol), second generation copying by a compliant recording device is still prevented.
Therefore in accordance with the present invention, there is a single class of watermark, which has two versions: copy-never and copy-once, which are identical except for a single bit. In accordance with the invention there is no need to add a watermark to the video upon recording, because instead the copy-once watermark is converted, by a simple one bit change, to the copy-never watermark. (xe2x80x9cWatermarkxe2x80x9d as used here means both watermarks and other types of digital xe2x80x9cfingerprints.xe2x80x9d) The present invention is applicable to both digital video and analog video (e.g. VHS, S-VHS or 8 mm video), as described below. The present watermark survives conversion from the analog to the digital domain or digital to analog domain, survives compression, and survives conversion between television standards i.e., PAL to NTSC or NTSC to PAL.
In accordance with the invention, a watermark, which is a digital signal, is conventionally embedded in a video image. A subset of the watermark bits carries a digital attribute (a number) which is a numeric characteristic of the video signal, for instance an average amplitude of the video signal over one video field or frame. The video attribute in accordance with this invention preferably is some unique characteristic of a video signal which changes maximally from frame to frame and which is not subject to the typical distortion present in the associated analog or digital transmission channel. The attribute is used only for the copy-once situation.
To prepare (encode) the video with the watermark, in addition to conventionally embedding the watermark, one also randomly (or pseudo-randomly) chooses one frame (or field), for instance one frame per every 10 seconds of video, and digitally calculates the attribute for that frame (or field) of the video signal. A xe2x80x9cfield markerxe2x80x9d (xe2x80x9ctagxe2x80x9d) is also added to that frame to indicate that it is a selected frame. This field marker is a special signal located in the normally invisible portion of the video frame, for instance in the overscan portion.
The encoded video signal is conventionally transmitted e.g. via cable television or satellite, to the consumer who attempts to record it using his compliant digital video recorder. The compliant digital video recorder prior to recording examines the watermark, verifies it, detects the copy-once bit in the watermark, and extracts the associated attribute value from the watermark. The compliant recorder also detects the field marker on a particular marked field, measures the attribute of that particular field, and compares the measured attribute to the extracted attribute value. If these two attribute values match, then the recorder performs the recording. If they do not match, recording (copying) is not enabled.
At the same time, the video recorder as it performs the recording strips away the field markers from the video so as to prevent any second generation copying of the resulting recording. The absence of the field markers prevents any second generation recording of a particular video signal by a compliant recorder.
Hence the compliant video recorder includes a watermark reader (detector and verifier), an attribute measurement circuit, a field marker remover (stripper) and associated intelligence (software) in the microprocessor conventionally resident in such a video recorder to perform the needed computations and logic functions including the attribute comparison.
Advantageously the field marker is a signal that cannot be detected and played back by conventional analog VCRs, since the field markers are chosen to be of a type removed by the chroma filtering circuitry conventionally present in such analog VCRs. Hence if the originally transmitted video signal is recorded using such a conventional analog recorder, the field markers are lost in the analog recording process, thus preventing any compliant digital recorder from making a second generation copy.
The field marker is typically inserted in the invisible portion of the active video, i.e. with regard to television sets in the overscan region. The field markers alternatively are located in the vertical or horizontal blanking intervals. The field marker is preferably a signal of a type automatically removed, as described above, by a conventional recording device, but this is not a requirement.
While a typical application of the present watermarking process is in the above-described cable television or satellite distribution system, this is not limiting. Usually of course prerecorded video material, for instance on tape or DVD (digital versatile disk), would not be subject to this process since normally such material is not subject to copy-once but is xe2x80x9ccopy-neverxe2x80x9d. However, the present copy-once watermarking process may be applied to prerecorded video material.